Electric cables with ethylene-propylene insulation

ABSTRACT

An electric cable comprises at least one insulated conductor enclosed in a sheath of a polymeric composition which comprises a polymerized propylene sequence and a propylene/ethylene copolymer sequence, has a melt index (230*C; 2.16 kgms) of 0.01 to 0.5, preferably 0.025 to 0.25 and an ethylene content in the range 1030 percent by weight, preferably at least 15 percent by weight. The conductor can be insulated with the same polymeric composition as is used for the sheath. The cable may also include a dielectric screen material which is the polymeric composition containing 20-40 percent by weight of carbon black.

United States'Patent 1 Verne et a1.

[541 ELECTRIC CABLES WITH ETHYLENE- PROPYLENE INSULATION [75] Inventors: Stefan Verne, London; Thomas Geoffrey Heggs, Welwyn, both of England [73] Assignee: British Insulated Callender & Ca-

- bles, Limited, London, England [58] Field of Search ..l74/23 R, 3 G, 107, 174/110 F, 106 SC, 110 AR, 1 PM, 113 R [56] References Cited UNITED STATES PATENTS Garner ...174/107 x 51 June 5, 1973 OTHER PUBLICATIONS Jones An Oil Extended EPR for Cable Insulations & Jackets in wire, Nov. 1966, pp. 1,822-1 ,826

Primary ExaminerE. A. Goldberg Attorney-Cushrhan, Darby & Cushman [57] ABSTRACT.

An electric cable comprises at least one insulated conductor enclosed in asheath of a polymeric composition which comprises a polymerized propylene sequence and a propylene/ethylene copolymer sequence, has a melt index (230C; 2.16 kgms) of 0.01 to 0.5, preferably 0.025 to 0.25 andan ethylene content in the range 10-30 percent byweight,

preferably at least 15 percent by weight. The conductor can be insulated with the same polymeric composition as is used for the sheath. The cable may also include a dielectric screen material which is the polymeric composition containing 20-40 percent by weight of carbon black.

A 8 Claims, 9 Drawing Figures PAIENTEUM 5m I 737 55.7

sum 1 [1F 2 PATENTEU JUN SIMS 7 SHEET 2 UF 2 This application is a division of our copending application, Ser. No. 70,987 filed Sept. 10, 1970.

This invention relates to olefine polymer compositions of propylene and ethylene and to the use of such compositions in electric cables.

Whilst the electrical properties of crystalline propylene/ethylene copolymers at present commercially available render these materials suitable for use in the manufacture of electric cables, especially as conductor insulation and/or sheathing, their excessive stiffness and excessive brittleness, especially at low and sub-zero temperatures, seriously limit their usefulness.

It is an object of the present invention to provide an improved crystalline propylene/ethylene copolymer having mechanical properties'that render the material eminently suitable for use in electric cables operating over a wide temperature range.

According to the present invention there is provided a crystalline polymeric composition comprising at least one sequence of either propylene homopolymerized or of propylene copolymerised with up to percent by weight copolymerized the composition of at least one other olefine monomer (which may be or may include ethylene), this sequence hereafter being referred to as the propylene sequence, and at least a further sequence of propylene copolymerized with ethylene, this sequence hereafter being referred to as the copolymer sequence, wherein said composition contains between 10 and 30 percent by weight of polymerized ethylene and has a melt flow index (measured by ASTM Test Method D 1238-65T at 230C using a 2.16 kgm weight) of between 0.01 and 0.5. The melt flow index of the polymeric composition is preferably between 0.025 and 0.25.

Preferably the composition comprises only one propylene sequence and only one copolymer sequence. The polymeric compositions may, however, comprise more than one of each sequence and in such a case the sequences will alternate with each other. The propylene sequence comprises the major part of the polymer composition and this sequence has a high softening temperature, comparable with that of crystalline polypropylene. The polymeric composition may contain sequences other than the propylene and copolymer sequences but these other sequences, for example a polyethylene sequence, may form only a minor part of the polymer composition and should not substantially affeet the properties of the polymer composition as a whole. 1f the propylene sequence is a copolymer the amount of comonomer present should be sufficient to improve the flexibility of the sequence compared to crystalline polypropylene without producing a pronounced lowering of the softening temperature of the polymeric composition. The amount of comonomer present in this sequence will be dependent on the particular comonomer or comonomers, and thus, if ethylene is used, only a small quantity should be present, preferably not more than 3 percent of the total weight of the polymeric composition, whilst if a higher olefine monomer is used such as butene-l, up to 10 percent of the total weight of the polymeric composition may be copolymerized butene-l without there being a significant effect on the softening temperature of the polymer composition.

The polymeric composition contains preferably at least 15 percent and particularly 25 percent by weight of polymerized ethylene. If both sequences contain ethylene, the total amount of ethylene should not exceed 30 percent by weight with no more than 3 percent by weight of ethylene being in the propylene sequence. 1t

will be appreciated that a reference to a polymeric composition containing ethylene or another olefine should be understood to mean that the monomer is present in the polymerized and/or copolymerized form.

According to a further aspect of the present invention there is provided a process for the preparation of a polymeric composition using a sequential polymerization technique wherein polymerization is effected in at least two stages, using a polymerization catalyst comprising titanium trichloride and an organ-aluminum compound, one of said stages being a polymerization stage giving a propylene sequence as hereinbefore defined, the other of said stages being a copolymerization stage giving a copolymer sequence as hereinbefore defined, the ethylene being polymerized in a quantity sufficient to produce a polymer composition containing 10-30 percent by weight of polymerized ethylene.

The titanium trichloride component of the catalyst can be any of the known forms of titanium trichloride. These include the product obtained by the reduction of titanium tetrachloride with hydrogen and the material described in British Pat. No. 877,050, which is the product obtained by the reduction of titanium tetrachloride with aluminum metal and which has the empirical formula AlTi cl However, we prefer to use as the titanium trichloride component the product of the reduction of titanium tetrachloride by an aluminum dialkyl halide at a temperature between 20C 'and +20C. We particularly prefer to use the product obtained by the gradual addition of an aluminum dialkyl halide to a stirred solution of titanium tetrachloride maintained at a'temperature in the range 20C to +20C-, preferably 0C. Using our particularly preferred titanium trichloride component together with a dialkyl aluminum halide as activator, it is possible to effect polymerization using high concentrations of monomers without obtaining a thick intractable slurry which would be difficult to process further.

Optionally, hydrogen is present during the propylene sequence polymerization stage. When hydrogen is used, it is present in a sufficient amount to produce a polymer composition having a melt flow index of 0.01 to 0.5 and we have found that polymer compositions having melt flow indices within the desired range can be obtainedat 60C using 0.05 to 0.125 mole percent of hydrogen in the propylene. However at lower and higher temperatures, greater and smaller quantities respectively of hydrogen will be required to obtain the desired melt flow index. At C, the amount of hydrogen which may be used is in the range 0.03 to 0.075 mole percent and at 53C, the maximum amount of hydrogen is 0.16 mole percent. At lower temperatures such as 30C or even 20C, the maximum amount of hydrogen to give the desired melt flow index is 0.27 or 0.33 mole percent respectively of hydrogen in the propylene.

When the polymerization is complete, the catalyst is deactivated, for example by the addition of an alcohol, and removed from the polymer using well known conventional catalyst removal procedures, for example, washing the polymer with water, more alcohol or an inert hydrocarbon such as the polymerization diluent. It is preferred in order to obtain good insulating properties, that the amount of residual catalyst is reduced to a low level.

Although the propylene sequence polymerization stage and the copolymer sequence copolymerization stage of the sequential polymerization may, if desired, be repeated several times, we prefer to effect each stage once only.

The polymerization can be effected by polymerizing propylene, optionally together with another olefine monomer as the first stage, and then, before the propylene in the first stage is completely polymerized, introducing, as a single shot, ethylene for the second stage and thereafter copolymerizing the ethylene and the propylene. The single shot of ethylene is in a sufficient amount to produce the desired quantity of polymerized ethylene in the final polymeric composition. Preferably the ethylene is introduced whilst there is a substantial quantity of propylene remaining, for example a partial pressure of propylene in excess of 0.5 atmospheres (0.53 kgm/sq cm). The ethylene is conveniently introduced when the partial pressure of propylene remaining is in the range 1.76 to 3.16 kgm/sq cm.

Alternatively, the second stage can be effected using a mixed feed of ethylene and propylene rather than a single shot of ethylene, and this technique produces polymer compositions having improved toughness compared to compositions produced by the use of a single shot of ethylene and containing the same amount of polymerized ethylene in the final product.

The polymeric compositions in accordance with the invention are tough, even at low temperatures and have heat deformation characteristics that approach those of polypropylene. Furthermore, the flexibility of our polymeric compositions as indicated by the modulus of elasticity is similar to that of high density polyethylene having a density of approximately 0.95 g/cm and the compositions exhibit high resistance to environmental stress cracking. Furthermore, the high melting temperature of our polymeric compositions permits their use at high temperatures. When suitably stabilized they can be used at such high temperatures for an extended period of time even when in contact with air and in intimate contact withcopper. For instance electric cables incorporating our compositions can be operated continuously at 90C for in excess of 20 years and would not be damaged at 145 for at least 1,500 hours.

An additional advantage of our polymeric compositions is that they show much reduced voiding in slow cooled samples. Thus, if a sample of crystalline polypropylene is heated on a microscope slide, and cooled slowly from the melt, small voids can be observed to form between the spherulites, whereas the polymeric compositions of the present invention show little or no voiding under the same conditions.

The mechanical properties of our polymeric compositions are combined with chemical inertness and a high degree of resistance to many environments including those in which fungi and bacteria are present and a high degree of resistance to insects such as termites. The polymeric compositions also have a high resistance to cutting, crushing and abrasion over a wide range of temperatures.

A further advantage of our polymeric compositions is that they will accept a higher loading of compounda sufficient conductivity but normally not more than 40 percent by weight and we have found that with 25 to 30 percent by weight of conductive carbon black a satisfactory conductivity is obtained without the mechanical properties of the polymeric composition being unacceptably affected. For these semi-conducting compositions it is preferred to use polymeric compositions containing 25 percent, or more, by weight of ethylene.

The combination of properties possessed by our polymeric compositions render them especially suitable for use in the manufacture of insulated electric conductors and cables and also for use as a semiconducting screen in cables.

The various forms of cables and conductors in which the polymer compositions may be used include low and medium voltage and super-tension power cables, control cables and covered wires, mineral insulated cables, pilot and telecommunication cables and radio and higher frequency cables. In additionto their use in cables and cable accessories, the mechanical and electrical properties of the polymeric compositions and the ease with which they can be extruded or otherwise processed render the materials useful for other electrical applications, for instance for insulating motor windings especially submersible pump motor windings.

Accordingly, the invention also includes an electric conductor having at least one insulating covering of solid, laminated or cellular form which covering is formed of a polymeric composition in accordance with the present invention. 'Ilhe invention includes an electric cable comprising a plurality of such insulated conductors. v

The invention further includes a single or multicore electric cable comprising an insulated conductor or a plurality of insulated conductors enclosed in a sheath of the polymeric composition of the present invention, the insulation of the conductors may also be made of the polymeric composition of the invention.

The invention also includes a single or multicore electric cable in which the or each core include a conductor and/or dielectric screen comprising a layer of a semi-conductive polymeric composition of the present invention containing conductive carbon black.

The polymeric compositions of the present invention can be applied to a conductor or core by extrusion over a wide range of wall thicknesses and can be successfully applied to both round and shaped conductors and to solid and corrugated metal sheaths, the technique employed being similar to that employed when extruding polyethylene. Insignia can be embossed on coverings of the polymeric composition using the technique hitherto employed when embossing polyethylene.

The improved mechanical properties of the polymeric compositions of the present invention will be further illustrated, by way of example, by the following test results that were obtained in testing two commercial grades of crystalline propylene/ethylene copoly- .5 mers designated A and'B, copolymer A being a high molecular weight extrusion grade material containing about 8 percent by weight of polymerized ethylene and copolymer B being an injection moulding grade material, containing about 15 percent by weight of polymerized ethylene, and four polymeric compositions, C, D, E and F in accordance with the present invention.

EXAMPLE 1 Preparation of polymeric composition 3 .70 l of a dry aliphatic hydrocarbon diluent boiling point l180C were added to a stirred jacketed stainless steel autoclave. All air was removed by evacuation and the system was purged with a stream of propylene for 5 minutes, 0.7 mole of diethyl aluminum .of ethylene were added over a period of 2 hours. Then the slurry was quenched with isopropanol, and catalyst residues were extracted in the normal fashion by continued washing with water. The polymeric composition was filtered and dried to yield 28 kg of powder, having a polymerized ethylene content of 25 percent by weight. This was densified in an extruder using 1,1,3- tris-(2'-methyl-5'-tert-butyl 4-hydroxyphenyl) butane and dilauryl thiodipropionate as stabilizers.

EXAMPLE 2 Preparation of polymeric composition A polymeric composition was prepared as in Example 1 except that the hydrogen concentration in propyl- I ene was 0.10 mole percent. The polymeric composition contained 25 percent by weight of polymerized ethylene.

. 6 merized ethylene. EXAMPLE 4 Preparation of polymeric composition F I Diluent, catalyst aluminum alkyl were charged into an autoclave, as in Example 1. 25.5 kg of propylene containing 0.05 mole percent of hydrogen were added over a period of 3hours. Then the autoclave'was cooled to C and propylene allowed to polymerize until its partial pressure in the reactor was 20 p.s.i.a. At this point 4.5 kg of ethylene were added over a period of 2 hours. After feeding 0.6 kg of ethylene, a continuous feed of propylene was also introduced at the rate of 2.3 kg/hr. After feeding all the ethylene, the reaction was quenched with isopropanol and worked up as in Example 1. The polymeric composition contained 15 percent by weight of polymerized ethylene.

A series of test specimens were prepared from polymeric compositions A to F by compression moulding granules of the compositions using a temperature of 200C for all polymeric compositions except composition A for which a temperature of 225C was used, a pressure of 1,100 p.s.i. for 0.25 inch thick samples or 630 p.s.i. for 0.125 inch thick samples and a moulding time of 10 minutes.

The samples were then either fast cooled by transferring to a press heated to 160C and cooling by circulating cold water through the press (to give a cooling time of 5-10 minutes) or slow cooled by leaving in the press and allowing to cool naturally, taking approximately 6 hours to cool from 200C to C.

The samples were then subjected to a series of tests to determine their mechanical properties. The results obtained with the fast cooled specimens are set out in Table I and with the slow cooled specimens in Table II. In Table III are set out the environmental stresscrack ing results obtained with both slow and fast cooled specimensfln Tables I and 111 results obtained on samples of high density polyethylene (HDPE) are also given by way of a further comparison.

Polymer Property A B C D E F HD P E Melt flow index (ASTM 1? 123 T) 0.4 1. 4 o, 05 Q18 Q19 Softening point C.) 0.1 inch penetration, 5 kg.lcm. load. 165. 5 1 7, 5 164 165 129 Vicat softening point BS 2782 but heating rate 120 C./hr 147(143) 133 12g 5 135 135 Izod notched impact strength, it./lb./in. notch ASTM D 256-56 Method A: C 15 15 2. a 9. 4 I3. 4. 8 14. 5 g. 3 1. 9 0 7. 7 Notched Charpy impact (1t.1b./rn. at 0 C.) 0.26 0. 21 2.05 1, 62 1 63 1,97 Cold shatter test T50 temperature C.) -23 Low temp. brittle point C. ASTM D 7461. 20 -39 70 Tensile properties, yield stress (lb-[1113).

2 inch s/min-lin h 3, 800 3, 200 2, 750 2, 45 000 2, 600 20 lnchesIminJinch 4, 400 3, 400 3, 000 900 3 100 2' 650 Break stress (1b./in.

2 lnchcslminJinch 5. 700 4, 200 4, 200 3, 7 300 5| 000 201nchcs/inln./inch 3. 500 2, 650 2, 300 g, m 2 950 3v 000 Elongation at hrak (percent):

2 lIIClIOS/lllllL/l neh 780 670 300 570 770 740 800 20 ineheslmlnJinch 740 520 4 0 720 I 610 800 1% sccant flcxural modulus (UL/m3) A IM I 7 045 100,000 103,000 113,000 123 133,000 8,000 130,000

I Specimens and specimen holder modified as in AS'IM Bulletin No. 231, .I My 1058. The samples 11 t in the standard 1) 7M equipment since only 15 specimens are loaded Into the small holder.

EXAMPLE 3 Preparation of polymeric composition E A polymeric composition was prepared, as in Example 1, except that 25.5 kg of propylene were charged initially, followed by 4.5 kg of ethylene. The polymeric composition contained 15 percent by weight of poly- T50 Temperature for 50 percent cold shatter failure) The specimen (5.5 X 0.2 X 0.125 ins), resting symmetrically on supports 2 inches apart in a thermos'tated alcohol bath was struck at its midpoint by a 1 lb wedgeshaped tup falling from a height of 24 inches. Multiple which failed to break was recorded and expressed as percentage pass. The temperature for 50 percent .pass wasdetermined froma-plot of percentage pass (on a log probability scale) against temperature).

The Charpy impact properties were measured using a ,I-lounsfield plastics impact tester manufactured by TensometerLtd, Croydon, to determine the Charpy impact strength (N..I.S.). In this impact strength test a specimen of'length 2 inches, width 0.25 inch and thick- .ness 0.125 inch having a V-shaped notch 0.11 inch ,deep with a tip radius of 0.01 inch cut in the middle of one of-the .2 inch X 0.25 inch sides, is supported at each end with its major axis at right angles to the path of a small pendulum .in such a position that the pendulum at the lowest'point of its path strikes the specimen at a 1 velocity of Y8 ft/second on the side opposite the notch and at a point directly behind it. The ends of the specimen are not clamped, but rest on two horizontal surfaces with the ends of the notched edge against rigid yertica'l stops. The average energy absorbed by 10 specimens is recorded.)

Tensile properties Yield stress (lblin 2 inches/min/inch 20 inches/min/inch Break Stress (lb/in) 2 inche's/min/inch 20 inches/min/inch Elongation at break 2'inc1ies/min/inch 720 I00 330 140 600 600 20 inches/min/inch 100 80 160 100 200 540 Note T Temperature for 50% cold shatter failure.

ventional copolymers A and B, especially with regard to the low temperature Izod impact strength and cold shatter tests on both slow and fast cooled mouldings. In fact polymeric compositions C,"D and F are tougher at 18C than high density polyethylene at roorn'temperature whereas most crystalline polypropylenes at present commercially available become brittle at; 0C.

All of the polymeric compositions C to F are more flexible than copolymers A and B and composition C is in fact marginally more flexible than high density polyethylene. The increase in flexibility of polymeric compositions C to F as compared with copolymers A and B has been achieved without asignificant reduc tion in the softening points of .the compositions, the softening points being approximately C higher than that of low density polyethylene andabout 35C higher than that of highdensity polyethylene. We have found that suitably stabilized polymeric compositions in accordance with the invention can be used continuously at C and for limited periods of time can be used at much higher temperatures, for instance for at least 1,500 hours at 145C.

In addition to the improved mechanical properties discussed above we have found that, unlike low and high density polyethylenes and commercially available polypropylenes the polymeric compositions of the present invention show an extremely high resistance to environmental stress cracking as illustrated by Table 111. Unlike high density polyethylenes they also shownothermal cracking.

It will be observed that copolymer B has the same ethylene content as polymericcompositions E and F but is less tough than compositions E and FJFurth'ermore, copolymer B is more liable to the formation of voids than either of compositions E and F. Equally, copolymer A, although having a melt flow index as specitied for our polymeric compositions, is also less tough than compositions C, D, E or F. Thus, the particular combination of properties possessed by compositions C, D, E and F is dependent on the polymeric compositions having an ethylene content and melt flow index within the specified range, materials having a lower ethylene content or higher melt flow index not possess- TABLE in ots-hometown s TREss CRAEKING [ASTM D 1693 procedure using 0.060 inch thick specimens] Environment at 65 C.

0.6 Cable oil Lissapol N Material F (0) Hours F (0) Hours F50 Hours From Hours Fast cooled 4, 000 0/10 300 1/20 500 10/20 800 20/20 v Slow cooled 4, 000 0/10 1,300 1/10 2, 000 5/10 2,200 10/10 Fast cooled 4, one one 4, 000 one I) Slow eoole( l 4,000 0/10 8,000 l/l0 4,000 2/10 I Fest rtoriletL, 4, 000 11/10 2, 000 l/ll) 4, 000 1/10 I slow ('(iOlld 4,0ee e/lu :i, see l/ll) 4, one l/lO Fest cooled. 4,000 0/10 1,000 0/10 I h'lowcouletl 4,000 0/10 4,000 0/10 I Fast cooled 4,000 0/10 1, 500 l/lll 4, 001) 3/10 Slow cooled 4, 000 0/10 3, 500 l/10 4, 000 2/10 High density polyethylene (fast cooled) 210 1/20 440 10/20 500 13/20 No'rEsEFt F and Fm refer to times to the lirst,5l)%end%i'ailures respectively. The ii zurt-s in lirnekel, lve the proportion of failures at the times shown. (1.6 Cable oil is a low viscosity hydrot-urlmn cable 011 sold by lJnsseks Ltd.

The a van g s p e y the polymeric p 5 ing the desirable combination of properties possessed tions of thepresent invention are apparent from Tables I to ill. The mechanical properties of polymeric compositions C, D, E and F are superior to those of the conby our polymeric compositions which makes them particularly suitable for use in cables.

Of the polymeric compositions C, D, E and F, com- EXAMPLE A semi-conducting composition was prepared con-- Melt Flow lndex (230C/10 kgm load) Tensile pro erties at extension rate of 200 per minute Yield stress 245 kgf/cm Yield extension 1 1.5% Elongation at break 380% D.C.resistivity (B.S.2044) 7 ohm.cm

When a similar semi-conducting composition was prepared using polymer instead of the polymeric composition C, the elongation at break was less than 10%.

EXAMPLE 6 A 660/1,100 volt power cable was made consisting essentially of three 194 mm (0.3 sq.in.), 120 sector shaped solid aluminum conductors insulated with a 0.9 mm (0.036 in.) radial thickness of impregnated paper. The three laid up' cores were enveloped by an aluminum tube of about'2.0 mm (0.08 in.) radial thickness performing the dual function of a neutral conductor and sheath. The outer surface of the concentric neutral conductor was treated with a suspension of zinc chromate in bitumen and the following protective layers were supplied by extrusion to four separate lengths of the cable: (i) a plasticized PVC of a grade typical of those used for cable sheating; (ii) a high density polyethylene (Hostalen GM-5010, sold by Hoechst); (iii) a polypropylene copolymer (Propathene PIPE-103, sold by 1C1) and (iv) polymericcomposition C in accordance with the present invention containing additives for protection against oxidative ageing and effect of sunlight. In each case the thickness of the extruded oversheath was .2.5 i 0.2 mm (0.099 t 0.008 in.) and the outer diameter about 42 mm (1.65 in.). The effectiveness of the protection given by the four materials is illustrated by the test results in Table 1V. The results show the unique combination of properties of the sheath of the polymeric composition of the present invention; absence of brittleness at -C combined with high resistance to cut propagation, abrasion, cutting and crushing.

TABLE IV High Test PVC density Polymer Polypoly- C propylene ethylene Resistance to cutting 1 alt 23C (kg) 74 500 500 500 at 65C (kg) 29 315 500 500 Resistance to crushing (2) i at 23C (kg) 1750 5000 5000 5000 at 65C (kg) 640 2300 5000 5000 Temperature of brittle failure under impact (3) T 5C 20C 30C -10C Resistance to abrasion (4) number of cutter revolutions 14 300 300 300 Resistance to cut propagation at 65C (5) Pass -Fail Pass Marginal The methods of test referred to in Table IV were carried out in the following manner. 1. Resistance to cutting. A 30 wedge, carrying 0.8 mm (0.032 in) wire on cutting edge, was advanced at 0.05 mm/min (0.002 in/min) against the cable sample supported on a flat anvil in a water-bath. The force on the wedge was recorded when the wedge made electrical contact with the aluminum outer (concentric neutral) conductor. 2. Resistance to crushing. A horizontal semi-cylindrical edge was advanced at 10 mm/min (0.394 in/min) against a cable sample supported on a semi-cylindrical anvil. The two semi-cylindrical tools were of approximately the same diameter as the cable, and were advanced with their axes parallel to one another and normal to the cable axis. Failure was detected electrically. 3. Temperature of brittle failure under impact. A horizontal semi-cylindrical striker of approximately the same diameter as the cable was loaded to 100 kg (220 lbs) and was dropped from 1.5 meter (4.92 ft) on to a cable sample pre-cooled to a specified temperature. The sample was supported on a flat anvil. 4. Resistance to abrasion. The resistance to abrasion was measured at room temperature, against a milling cutter with 16 teeth, ground back to angles of 60 and 20 to a tangent. A load of 50 kg (1 10 lb) was applied in a direction normal to the axis of the cutter which was caused to rotate at a peripheral speed of 25 mm/sec (0.984 in/sec). The number of cutter revolutions required for the cutter to wear through to the outer conductor was measured.

5. Resistance to cut propagation. A circumferential cut,

1.25 mm (0.05 in) deep, was made in the cable sample by a suitably mounted razor blade. The sample was then bent to fit a former of 45 cm (17.7 in) radius and immersed in water and was subjected to 21 daily temperature cycles of 16 hours at C and 8 hours at 23C.

EXAMPLE 7 A 20 pair telephone distribution cable was made containing 40 copper conductors, each of 0.9 mm (0.036 in) diameter and insulated with 0.3 mm (0.012 in) radial thickness of a conventional low density polyethylene. One length of this cable was sheathed with 3 mm (0.1 18 in) radial thickness of a conventional low density polyethylene and another with the same thickness of polymeric composition C in accordance with the invention containing additives for protection against sunlight and oxidative ageing. A test, carried out to assess the degree of protection against damage, consisted of laying a sample of cable on a typical bed" (i.e. compressed sand) and dropping a spade, loaded to 45.34 kg lb) from predetermined heights. When the spade was dropped from a height of 191 mm (7.5 in), the length sheathed with polyethylene was severely damaged with the sheath cut and some conductors severed.

The length sheathed with the polymeric composition C withstood the spade being dropped from 254 mm in) without either the sheath or conductors being cut; when the spade was dropped from 356 mm (14 in), the conductors were still unbroken although the sheath was cut. Hitherto, cables of this type have been protected by steel armor in order to achieve resistance to cutting by a spade dropped from a height of 191 mm (7.5 in).

EXAMPLE 8 An 11,000 volt power cable consisted essentially of three 185 mm (0.286 sq in) circular stranded aluminum conductors, each covered with successive layers of semi-conductive screen, insulation and dielectric screen. The construction of the cable is as follows:

Conductor diameter Semi-conducting screen of Example 5, thickness Insulation (Polymeric composition C compounded with antioxidants and a copper inhibitor), thickness Semi-conducting screen of Example 5, thickness Core diameter Laid-up Cores Diameter Binder thickness (polypropylene tape) 2 copper tapes 0.0762 mm (0.003 in) thick, 40 mm (1.575 in) wide applied with 50% overlap Diameter over copper tapes Oversheath (Polymeric composition C stabilized against deleterious effect of sunlight and compounded with antioxidants and a copper inhibitor), thickness Oversheath diameter 17.6 mm (0.693 in) 0.5 mm (0.020 in) 3.4 mm (0.134 in) 0.7 mm (0.028 in) 26.8 mm (1.055 in) 58.5 mm (2.3 in) 1.0 mm (0.039 in) 59.8 mm (2.36 in) 2.8 mm (0.110 in) 65.4 mm (2.58 in) EXAMPLE 9 The crystalline propylene/ethylene copolymer of the present invention, for instance polymeric composition C, is used as the solid plastics dielectric of the or each core of a high voltage electric power cable for installation in a system in which during operation of the cable the plastics dielectric is subjected to a pressure above atmospheric by a cable gas, such as nitrogen or sulphur hexafluoride, which has access to one or more surfaces of the dielectric. The copolymer of the dielectric is impregnated with a cable gas to an extent such that, until the cable is put into service, at least the major part of the solid dielectric remains fully impregnated with the gas, thatis to say any void in the dielectric or between the dielectric and a conductive body bonded thereto, is filled with the cable gas at a pressure of at least lbs/sq.in, preferably at least 30 lbs/sq.in.

EXAMPLE 10 A mineral insulated cable comprising one or more conductors insulated from each other and from an aluminum sheath by compacted magnesium oxide has an oversheath of polymeric composition C compounded with antioxidants and preferably but not necessarily containing 2% percent of carbon black as protection against the effect of sunlight to protect the aluminum sheath against corrosion.

Use of the polymeric compositions of the present invention in insulated electric conductors and cables will be further illustrated by descriptions, by way of example and with reference to the accompanying drawings which show cross-sectional views of various forms of electric cable incorporating the polymeric. compositions as conductor insulation, as semi-conducting screens and/or as cable sheaths.

In the accompanying drawings:

FIGS. 1 and 2 show two forms of telephone distribution cable;

FIGS. 3, 4, 5 and 6 show four forms of low voltage power cable;

FIGS. 7 and 8 show two forms of medium voltage power cable, and

FIG. 9 shows a single core medium voltage cable.

The telephone cables shown in FIGS. 1 and 2 each comprise a multiplicity of plastics insulated conductors 1, surrounded by one or more layers 3 of helically lapped paper tape and enclosed by an outer protective sheath 4. The interstices between the insulated conductors l and between them and the layers 3 of paper tape are preferably filled with a water-impermeable medium such as petroleum jelly compound 2 which also impregnates the paper tape. In the telephone cable shown in FIG. 1 the sheath 4 is of composite form comprising an inner layer 5 of polyethylene and an outer layer 6 of a crystalline propylene/ethylene copolymer in accordance with the invention, such as polymeric composition C. The sheath 4 of the telephone cable shown in FIG. 2 comprises a single extruded layer 7 of the-crystalline propylene/ethylene copolymer. In each cable the plastics insulation of the conductors may be made of a crystalline propylene/ethylene copolymer or it may be of another polymeric material such as polyethylene; the conductor insulation may be of solid or cellular form. If desired the layers of impregnated paper tape may be replaced by a layer of longitudinally applied aluminum tape having at least on its outer surface a coating of polyethylene.

The low voltage power cable shown in FIG. 3 consists essentially of three sector-shaped solid aluminum conductors 11, each covered by a layer of insulation 12, to form th'ree insulated cores. The three cores are laid up together and are surrounded by one or more layers 14 of binding tape, an extruded aluminum sheath l5 and an outer sheath 16 of a crystalline propylene/ethylene copolymer in accordance with the invention such as polymeric composition C. The insulation 12 of each conductor may be an extruded layer of a crystalline propylene/ethylene copolymer or of another polymeric material or it may comprise layers of helically lapped impregnated paper tape.

An alternative form of low voltage power cable shown in FIG. 4 comprises four sector shaped solid aluminum conductors, three of the conductors 21 constituting the power conductors and the fourth conductor 23 constituting the neutral conductor. Each conductor is covered by an extruded layer 22 of insulation. The four cores are laid up together and surrounded by a bedding 25 for a layer of armoring 26, which may comprise helically lapped steel wire or aluminum strip, and an outer sheath 27 of a crystalline propylene/ethylene copolymer of the present invention such as polymeric composition C. As n the example shown in FIG. 3 the insulation 22 of the conductors 21 or 23 may be of a crystallinepropylene/ethylene copolymer or of another polymeric material.

FIG. shows a low voltage power cable comprising three solid aluminum power conductors 31, each insulated with an extruded layer 32 of a crystalline propylene/ethylene copolymer of the present invention, and a solid aluminum neutral. conductor 33 enclosed in a covering 34 of extruded lead. The four cores are laid up together and are surrounded by one or more layers 35 of helically lapped steel tape and an outer protective sheath 36, of polymeric composition C.

The low voltage cable shown in FIG. 6 comprises three sector shaped solid aluminum power conductors 41 each insulated with a layer 42 of polymeric composition C. The three cores are laid up together and are surrounded by a concentric neutral conductor 44 of stranded aluminum wires, the direction of lay of which is reversed at intervals along the length of the cable, which is embedded in anti-corrosion bedding 43. The assembly is enclosed in a sheath 45 of polymeric composition C.

FIG. 7 shows an 11,000 volt power cable consisting essentially of three 185 mm (0.286 sq.in) sector shaped solid aluminum conductors 51 each covered with successive layers of semi-conductive screen 52 insulation 53 and dielectric screen 54. The construction of the cable is as follows: i

Semi-conducting screen 52 of Example 5 thickness Insulation 53 (Polymeric composition C compounded with anti-oxidants and a copper inhibitor),-thickness Semi-conducting screen 54 of Example 5, thickness Laid up 'cores diameter Bedding 55 of semi-conducting rubber-like material, overall diameter Armouring Layer 56 of helically wound steel wires (2.5 mm (0.099 in) in diameter).Diameter over armouring She'ath 57 (Polymeric composition C stabilized against deleterious effect of sunlight and compounded with anti-oxidants and a copper inhibitor) thickness Overall diameter 0.5 mm (0.020 in) 3.4 mm (0.134 in) 0.5mm (0.020 in) 46.1 mm (1.82 in) 48.1 mm 1.89 in) 53.1 mm (2.01 in) 2.2 mm (0.087 in) 57.94 mm (2.28 in) formed by one or more layers 65 of helically lapped copper tape. The three screened cores are laid up together with fillers 66 and are bound by a helical lapping 67 .of insulating tape. A sheath 68 of polymeric composition C surrounds the assembly of cores.

FIG. 9 shows a single core power cable comprising a solid aluminum conductor 71, a conductor screen 72 formed by an extruded layer of the semi-conducting polymeric composition described in Example 5, an extruded layer 73 of polymeric composition C constituting the conductor insulation, a dielectric screen 74 of the same material as the conductor screen 72 and a layer 75 of stranded tinned copper wires.

Although in each of the cables described with reference to FIGS. 3 to 9 the core conductors are described as solid aluminum conductors each may alternatively be formed of stranded aluminum or copper wires.

--We claim:

1. An electric cable comprising at least one insulated conductor enclosed in a sheath formed of a crystalline, polymeric composition comprising at least one sequence of either homopolymerized propylene or of propylene copolymerized with up to 10 percent by weight of the composition of at least one other olefine monomer which may be or may include ethylene and at least a further sequence of propylene copolymerized with ethylene wherein said composition contains between 10 and 30 percent by weight of polymerized ethyleneand has a melt flow index of between 0.01 and 0.5.

2. An electric cable as claimedin claim 1, wherein the crystalline polymeric composition of the sheath has a melt flow index of between 0.025 and 0.25, an ethylene content of at least 15 percent by weight and consists of one sequence of homopolymerized propylene and one sequence of propylene copolymerized with ethylene. I

3. An electric cable as claimed in claim 1', wherein the insulating covering of .the conductor or of at least one of the conductors is formed of a crystalline polymeric composition comprising at least one sequence of either homopolymeri'zed propylene or of propylene copolymerized with up to 10 percent by weight'of the composition of at least one other olefine monomer which may be or may include ethylene, and at least a further sequence of propylene copolymerized with eth- -ylene wherein said.composition .containsbetween l0 and 30 percent by weight of polymerized ethylene and has a melt flow index of between 0.01 and 0.5. A

4. An electric cableas claimed in claim 1 wherein the or each insulated conductor has a conductor-and/or dielectric screen formed of a composition comprising a. a crystalline polymeric composition having at least one sequence of eitherthomopolymerizedpropylene or of propylene copolymerized with up to. 10 percent by weight of the polymericc omposition of at least one other olefine monomer which may be e or may include ethylene, and at least a further sequence of propylene copolymerized with ethylene wherein said polymeric composition contains between 25 and 30 percent by weight. of polymerized ethylene and has a melt flow index of between 0.01 and 0.5 and 1 b. 2040 percent by weight, basedon the total composition, of conductive carbon black.

5. An electric cable as claimed in claim], wherein a plurality of plastics insulated conductors are enclosed within the sheathand the interstices between the insulated conductors and between them and the sheath from end to end of the cable length are filled with a water impermeable medium.

6. An electric cable as claimed in claim 5, wherein the plastics insulating covering of each conductor is of cellular form.

7. -An electric cable as claimed in claim 1,.wherein the insulating covering of them each conductor is the insulating covering of the or each conductor is subgas to an extent such that until the cable is puttintolserjected to a pressure above atmospheric by a cable gas vice, at least the major part of the solid insulating covwhich has access to one or more surfaces of the dielecering remains fully impregnated with'the gas. tric, wherein the insulating covering of the or each conl I ductor is of solid form and is impregnated witha cable :u

2333 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,737,557 Dated June 5, 1973 Invenc -(s) Stefan Verne et 31.

It is certified that error appears in the aboveidentified patent and that said Letters Patent: are hereby corrected as shown below:

In the heading of the patent, add the priority data as follows:

[30] Foreign Application Priority Data September 11, 1969 Great Britain. 45000/69- Change the assignee from "British Insulated Callender &

Cables, Limited" to --British insulated Callender's Cables Limited I Signed and Scaled this twenty-sixth ay f August 19 75 [SEAL] Arrest:

:UTH. C. MiSON} C. MARSHALL DANN Nesting 0/]!( Commissioner uj'PaIents and Trademarks Q UNITED STATES PATENT OFFICE QERTEFICATE OF CORRECTIGN Patent No. 3,737,557 Dated June 5, 1973 m- *1 r r a 1 Iuventor(s) \NBIYLQF it til,

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the patent, add the priority data as follows:

[30] Foreign Application Priority Data September 11, 1969 Great Britain.... 45000/69- Change the assignee from "British Insulated Callender &

5 Cables, Limited" to British insulated Callenders Cables Limited-- Signed and Scaled this twent -sixth a [SEAL] J D Of August1975 Arrest:

RUTH C. MQAHSON C. MARSHALL DANN Anestmg 01/16 ('mnmissimur ufParems and Trademark a L J 

2. An electric cable as claimed in claim 1, wherein the crystalline polymeric composition of the sheath has a melt flow index of between 0.025 and 0.25, an ethylene content of at least 15 percent by weight and consists of one sequence of homopolymerized propylene and one sequence of propylene copolymerized with ethylene.
 3. An electric cable as claimed in claim 1, wherein the insulating covering of the conductor or of at least one of the conductors is formed of a crystalline polymeric composition comprising at least one sequence of either homopolymerized propylene or of propylene copolymerized with up to 10 percent by weight of the composition of at least one other olefine monomer which may be or may include ethylene, and at least a further sequence of propylene copolymerized with ethylene wherein said composition contains between 10 and 30 percent by weight of polymerized ethylene and has a melt flow index of between 0.01 and 0.5.
 4. An electric cable as claimed in claim 1, wherein the or each insulated conductor has a conductor and/or dielectric screen formed of a composition comprising a. a crystalline polymeric composition having at least one sequence of either homopolymerized propylene or of propylene copolymerized with up to 10 percent by weight of the polymeric composition of at least one other olefine monomer which may be or may include ethylene, and at least a further sequence of propylene copolymerized with ethylene wherein said polymeric composition contains between 25 and 30 percent by weight of polymerized ethylene and has a melt flow index of between 0.01 and 0.5 and b. 20-40 percent by weight, based on the total composition, of conductive carbon black.
 5. An electric cable as claimed in claim 1, wherein a plurality of plastics insulated conductors are enclosed within the sheath and the interstices between the insulated conductors and between them and the sheath from end to end of the cable length are filled with a water impermeable medium.
 6. An electric cable as claimed in claim 5, wherein the plastics insulating covering of each conductor is of cellular form.
 7. An electric cable as claimed in claim 1, wherein the insulating covering of the or each conductor is compacted powdered mineral insulation and a metal sheath underlies the sheath formed of the crystalline polymeric composition.
 8. An electric cable as claimed in claim 3, for installation in a system in which during operation of the cable the insulating covering of the or each conductor is subjected to a pressure above atmospheric by a cable gas which has access to one or more surfaces of the dielectric, wherein the insulating covering of the or each conductor is of solid form and is impregnated with a cable gas to an extent such that until the cable is put into service, at least the major part of the solid insulating covering remains fully impregnated with the gas. 